The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the South Atlantic Ocean, consisting of nine main segments, E1 (north) to E9 (south). Major and trace element and Sr–Nd–Pb isotope compositions are presented, together with water contents, for lavas sampled along the active ridge axis. Magmatism along the East Scotia Ridge is chemically heterogeneous, but there is a common mid-ocean ridge basalt (MORB)-type source component for all the magmas. An almost unmodified MORB-source mantle appears to underlie the central part of the back-arc. Subduction components are found at the northern and southern ends of the ridge, and there is a marked sediment melt input of up to 2% in segment E4. Enriched (plume) mantle is present beneath segment E2 at the northern end of the ridge, suggesting that plume mantle is flowing westward around the edges of the subducting slab. The southern part of segment E8 is unique in that its magma source is similar to sub-arc depleted mantle.